For many years, renewable and alternative sources of energy have been explored to a large extent, and during recent years, there has been an ever increasing awareness of the overall energy generation and consumption prompted in particular by global climate change concerns and higher prices on traditional resources such as oil.
One such renewable source is solar energy. Solar energy utilizes the radiation from the sun to generate heat or power, basically either by generating electrical power directly, i.e. by converting the solar radiation into direct current electricity using photovoltaic solar panels comprising a number of solar cells containing a photovoltaic material, or by means of solar thermal collectors to utilize the thermal energy to generate heat, for instance for residential heating purposes, for evaporation of liquids (distillation) or to generate electricity by heating water or another heat transfer fluid to produce steam which in turn drives a turbine coupled to an electrical power generator.
One way of obtaining very high temperatures and thus an efficient utilization of the solar thermal energy is concentrated solar power (CSP) systems, which fundamentally use mirrors or lenses to concentrate a large area of sunlight, i.e. solar thermal energy, onto a small area.
Concentrating technologies exist in four basic types, viz. solar collectors in the form of parabolic troughs, dish mirrors, concentrating linear Fresnel reflectors, and plane solar collectors directed towards a solar power tower. CSP has recently been widely commercialized and has gained a substantial market share of the renewable sources. Of the CSP plants world-wide, parabolic-trough plants account for major part.
In principle, a solar thermal energy collector comprising a parabolic trough consists of a linear parabolic reflector that concentrates radiation onto a receiver positioned along the focal line of the reflector. The receiver is a tube positioned directly above the middle of the parabolic mirror and filled with a heat transfer fluid. The parabolic trough is usually aligned on a north-south axis, and rotated to track the sun as it moves, or the trough can be aligned on an east-west axis, which reduces the need for tracking but reduces the overall efficiency of the collector.
The parabolic trough reflector is traditionally formed as an elongated parabolic mirror which is carried by a support structure. The mirror is traditionally made up of glass, which has excellent reflecting and stability properties. However, glass has some disadvantages. For instance, it is fragile, meaning that particular precautions must be taken during manufacture, transportation and installation. Furthermore, the weight of such mirrors is substantial, which poses great demands on the support structure, and the overall costs of manufacture, transportation, installation, and maintenance of the system are relatively high.
In the prior art, attempts have been made to provide alternatives to such assemblies of parabolic mirrors. Examples are shown in US published patent applications No. 2010/0313933 and EP patent No. 338 051. Another example is given in for instance US published patent application No. US 2006/0193066 A1, which suggests the formation of reflective cones as an alternative to solar collectors in the form of parabolic troughs and dish mirrors. One of the advantages associated with the particular design mentioned in this document is that the considerable stability required constructing and maintaining parabolic troughs and mirror dishes may be avoided. However, the efficiency of a parabolic trough or mirror dish is considerably larger than with the solar collector suggested in this document.
In known plants, reflectors made of aluminium sheet are used to reduce weight. However, the reflecting properties of aluminium are lower than those of glass; furthermore, as such solar collectors are exposed to weathering at all times, bare aluminium needs frequent cleaning and polishing in order to retain its reflecting properties.
Document WO 2007/096158 A1 describes a self-supporting support structure in the form of hollow chamber profiles carrying on its front face a parabolic concentrator reflector. The support structure is provided as profiles that are extruded, strand-drawn or roll-formed. Similarly, US2008308094 describes a method of providing a self supporting trough-type solar unit where the supporting structure of transverse ribs surrounding the reflective element. Although this design alleviates many of the disadvantages of other alternative assemblies, such design is not very flexible in and hence there is still room for improvement with regard to ease of production, transportation, installation, flexibility of size, maintenance and also very much the utility rate of reflector units.